(1) Field of the Invention
The present invention relates to a paste for forming a transparent conductivce coating film of metal oxide form on glass and other substrates by screen printing and baking.
(2) Description of the Prior Art
It is known that a coating film of an oxide of Cd, In, Sn, etc. formed on glass and ceramic substrates is transparent and highly conductive. On account of this property, it is used for semiconductor devices, liquid crystal display units, window deicing conductive film, etc., and its potential uses are expanding. Such transparent conductive films are made of metal oxide are formed by chemical spray, vacuum deposition, or screen printing processes.
The chemical spray process is advantageous for forming a coating film of comparatively large area, but is disadvantageous in that it requires an additional etching step for complicated configurations. The vacuum deposition process does not need etching owing to the recently developed mask deposition process, but is not suitable for mass production because it is carried out batchwise. The screen printing process is free of these disadvantages; it is advantageous in that any configuration can be formed by printing and baking and it does not give off liquid waste resulting from etching. Nevertheless, the screen printing process has been poor in workability because the paste for forming transparent conductive coatings is poor in stability, and the coating films formed by this process are not uniform in conductivity.
Heretofore, the paste was composed mainly of organic acid indium salt such as indium octylate [(C.sub.7 H.sub.15 CO.sub.2).sub.3 In)] having a strong ionic bond. This compound, however, is susceptible to hydrolysis and promotes gelation of paste when made into a paste. This chemical instability shortens the life of paste and impairs the workability of screen printing. In addition, the resulting coating film has a comparatively high resistance and a low strength because the paste contains ethyl cellulose as a binder which is excessively heat-resistant (e.g., it does not burn completely at 500.degree. C.). In order to overcome these disadvantages, the present inventors successfully used an organic indium complex such as trisacetylacetonatoindium (III) [In(acac).sub.3 ] with acetylacetone (Hacac) coordinated therein as a compound forming metal oxide film on baking, and an organic tin complex such as dimethyltin acetylacetonate [(CH.sub.3).sub.2 Sn(acac).sub.2 ] as a tin compound for resistance adjustment to improve the paste stability. Moreover, the present inventors have found that the adhesion of coating film is improved and the resistance of coating film is lowered if nitrocellulose is used as a thickener. However, a nitrocellulose-containing paste is shorter in life and poorer in printing workability as compared with an ethylcellulose-containing paste. It has now been found that these drawbacks are alleviated to a considerable extent by using deacidified nitrocellulose.